Are you having problems like I was trying to upload files onto the CEAC website?
Here's how I was finally able to make my multi page pdf files (e.g. I-864) small enough to upload:
Step one: convert pdf to jpeg files (page by page, very tedious).
Step two: adjust the size of each jpeg (again, page by page)
Step three: make a new pdf using the new jpegs
Step four: save a compressed version of the pdf
I used Mac's program "Preview" and Adobe Acrobat